In the conventional process of soft hydrated contact lens manufacture, a key step is polymerisation of a liquid monomer in a cast mould. The mould comprises a male and a female mould part which, when engaged, form a lens-forming recess. Liquid monomer is poured into the female mould part before engagement of the male part with the female part. To ensure that the lens is correctly formed, the volume of liquid supplied is greater than the volume of the lens-forming recess. The excess liquid is displaced into an overflow cavity defined between the mould parts, the overflow cavity being disconnected from the lens-forming recess when the mould parts are fully engaged. The polymerisation results in a brittle lens (in the lens-forming recess) and a surrounding ring of brittle excess polymer known as xe2x80x9cflashxe2x80x9d (in the overflow cavity). Removal of the lens from the mould can result in the lens and flash ring becoming mutually electrostatically attracted. This is a problem during subsequent hydration of the lens when the flash ring can become adhered to the lens. The problem is exacerbated by the fact that removal of the lens from the mould often results in fragmentation of the flash ring. Contamination of lenses by small fragments of flash during hydration accounts for a significant proportion of wastage in the manufacture of contact lens.
It is an object of the present invention to provide an improved mould for use in the manufacture of a contact lens which obviates or mitigates the above-mentioned problem.
According to a first aspect of the present invention there is provided a cast mould for use in the manufacture of a contact lens by the polymerisation of a flowable precursor comprising:
(i) a female mould part having a concave first surface region adapted to receive the flowable precursor in use, and a second surface region; and
(ii) a male mould part having a convex first surface region and a second surface region, the male mould part being engageable with the female mould part to define a lens-forming recess between the concave and convex surface regions and to define a first overflow cavity between said second surface regions, the first overflow cavity being adapted to receive excess flowable precursor displaced from the lens-forming recess in use; wherein the first overflow cavity is provided with first retaining means such that, in use, after polymerisation, polymer in the first overflow cavity is mechanically interlocked with a single one of the mould parts after separation of the mould parts.
Preferably, the first retaining means is integrally formed with the second surface region of one of the mould parts. More preferably, the first retaining means comprises at least one recess in the second surface region and/or at least one protrusion extending from the second surface region into the first overflow cavity in use.
Preferably, the first overflow cavity is annular and extends around the lens-forming recess, in which case said at least one recess is preferably an annular undercut in said second surface region and said at least one protrusion is preferably an annular rib.
Preferably, the first retaining means is provided in the second surface region of the female mould part.
Preferably, the male and female mould parts each have a third surface region which define a second overflow cavity therebetween when the parts are engaged, the second overflow cavity being adapted to receive, in use, any excess flowable precursor displaced from the first overflow cavity, said second overflow cavity being provided with second retaining means such that after polymerisation, polymer in the second overflow cavity is mechanically interlocked with a single one of the mould parts after separation of the mould parts.
More preferably, the first and second retaining means are formed in the second and third surface regions respectively of the same (most preferably female) mould part.
The present invention also resides in the use of the mould in a method of manufacture of a contact lens, preferably a soft hydrated contact lens.